The present invention pertains to detection of particle contamination on an optical surface, and in particular to in-situ measurement of particle contamination on optical surfaces.
High quality, precision optical equipment often cannot perform to its optimum capabilities when its surfaces are contaminated with particles, such as dust. For example, one possible requirement of optical space surveillance sensors is to detect low-radiance targets in the presence of intense out-of-field sources, such as the Earth and sun. This requirement places high demands on system capabilities requiring very low scatter optical surfaces to be used for elements of the optical path. Contamination particles which are deposited on the optical surfaces during fabrication, sensor testing, storage, launch, deployment or operation, may significantly reduce system performance resulting in reduced signal levels and unacceptable signal to noise performance due to the scattering and extinction of light caused by the contamination particles.
One technique used to measure particle contamination on an optical surface utilizes a strip of adhesive tape to lift particles off the optical surface. The size and distribution of particles are then evaluated under a microscope. One disadvantage to this technique is that as the optical system comes together during the assembly phases, the optical surface becomes inaccessible. Another disadvantage to this technique is that it cannot be used in the field or on remote platforms, such as optical space surveillance sensors or space based telescopes.
Another technique used to measure particle contamination on an optical surface uses a detector to measure light scattered from the contamination particles on the optical surface. A light source illuminates a portion of the optical surface and either the light source or the detector is moved to measure the scatter from the particles at various angles. The angular dependence of the scatter is used to estimate the particle contamination level of the optical surface. One problem with this technique is that it can be used to measure the particle contamination of only one optical surface and is not particularly useful in systems that include several optical surfaces. Another problem with this technique is that it does not distinguish very well between scatter cause by particle contamination and scatter caused by micro-roughness of the optical surface. Another problem with this technique is that it is difficult to suppress spurious stray light paths that can invalidate the scatter measurements.
Thus, there is general need in the art for an improved system and method for determining particle contamination levels on optical surfaces. There is also a general need in the art for a system and method for determining particle contamination on inaccessible optical surfaces of optical systems such as remotely located space surveillance sensors or remote platforms, or on satellites with optical sensors or telescopes. There is also a general need in the art for a system and method for determining particle contamination on each of the various optical surfaces of systems that employ several optical surfaces.
The need in the art is addressed by the various embodiments of the present invention which provide a system and method for measuring particle contamination on optical surfaces. In one embodiment, the system includes a detector array, a non-coherent light source that illuminates the detector array with non-coherent light reflected by the optical surfaces, and processing equipment that analyzes shadows on the detector array. The shadows are indicative of particle contamination of the optical surfaces. In this embodiment, the system also includes a light source controller that changes the position of the non-coherent light source. The processing equipment distinguishes shadows caused by particle contamination on a first of the optical surfaces from shadows caused by particle contamination on the other optical surfaces based on movement of the shadows. The system also includes a particle contamination level analyzer that estimates an average particle contamination level for the optical surfaces based on contrast levels of the shadows identified for each optical surface.
In another embodiment of the present invention, a system for measuring particle contamination on refractive optical surfaces is provided. In another embodiment, a plurality of non-coherent light sources separately illuminate the detector array to produce sets of shadows on the detector caused by particle contamination on the each of the optical surfaces. In this embodiment, the shadows of a first set are distinguished from shadows of a second set by identifying different changed positions for the shadows which result from separate illumination of each non-coherent light source of the plurality.
In yet another embodiment of the present invention, a method for measuring particle contamination levels on optical surfaces is provided. The method includes illuminating a detector array with light reflected or refracted by the optical surfaces from a non-coherent light source, and identifying shadows on the detector array. In this embodiment, the method includes changing a position of the non-coherent light source to distinguish shadows caused by the particles on each of the optical surfaces, and estimating an average particle contamination levels for each of the optical surfaces based on contrast levels of the shadows identified for each optical surface.